Electrolytic plating of aluminum cannot be conducted in an aqueous system because the affinity of aluminum to oxygen is strong, and the electrolytic potential thereof is baser than hydrogen. Therefore, electrolytic plating of aluminum is conducted in a non-aqueous medium, especially in an organic medium.
Typical among the known organic electrolytic baths for aluminum plating are a bath comprising AlCl.sub.3 and LiAlH.sub.4 or LiH dissolved in ether and a bath comprising AlCl.sub.3 and LiAlH dissolved in tetrahydrofuran (THF). However, these baths contain highly reactive LiAlH.sub.4 and LiH, which react with oxygen or moisture, which may be contained in the bath, and may decompose thus deteriorating the electric current efficiency and shortening the bath life.
In order to overcome these disadvantage of the prior art, bath compositions comprising an aluminum halide and an onium salt of a nitrogen-containing heterocyclic compound have been proposed. These include bath compositions obtained by melting and mixing an aluminum halide and an N-alkylpyridinium halide (Japanese Laid-Open Patent Publication No. Sho 62-70592 and Sho 62-70593), a bath composition obtained by melting and mixing an aluminum halide and a 1-alkyl- or 1,3-dialkylimidazolium halide (Japanese Laid-Open Patent Publication No. Hei 1-272790), etc.
These bath compositions are liquid at room temperature and free from the danger of ignition, and if an aluminum anode is used, the bath is continuously replenished with aluminum as the aluminum in the bath is consumed. These bath compositions are more advantageous than other bath compositions in that the maintenance of the bath is easy and thus the operation is simpler.
Recently, electrolytically aluminum-plated products having a thick aluminum coating of no less than 10-50 .mu.m, inter alia, anodized products, are attracting attention as corrosion-resistant materials.
When thick aluminum plating is conducted using any of the above-described bath compositions comprising an aluminum halide and an onium salt of nitrogen-containing heterocyclic compound, however, aluminum is not uniformly deposited but particles thereof grow locally larger, making the surface irregular, and that the particles on the surface may come off under friction. The irregularity of the surface spoils the (surface) luster and, therefore, such products are not suitable to be used as reflectors or the like.
One measure for overcoming the above problems is to add an organic solvent such as benzene, toluene, or the like to the bath in an amount of 1-2 moles per mole aluminum halide. However, addition of such a large amount of organic solvent is not preferable because the solvent deteriorates the working environment by evaporation thereof and, moreover, invites danger of ignition.
Further, the above-described bath compositions are inferior in covering powder and, therefore, when shaped bodies are plated, concaved parts where the current density is not more than 0.01 A/dm.sup.2 may not be plated.
The present invention provides an electrolytic aluminum plating bath composition which enables overall, uniform, dense and smooth plating with low current density.
We have found that the above-described problems can be overcome by addition of specific organic heterocyclic compounds, and organic polymers, if desired, to the above-described bath compositions.